Throughout their history, attempts have been made to increase the efficiency of internal combustion engines. Although many designs and alternatives have been proposed, it is generally conceded that for the foreseeable future, the spark ignition and diesel designs will be the motive engines of choice.
Mass produced engines have relatively mediocre efficiency ratings--about 35-40%. The great bulk of wasted energy is lost in the form of unutilized heat. Accordingly, engine development has been directed towards increasing the recovery and utilization of the lost heat.
In particular, in recent years there has been extensive research directed towards adiabatic engines (more properly low heat rejection engines) and methods of recovering extra power from them. These are best summarized in Comparative Evaluations of Three Alternate Power Cycles for Waste Heat Recovery from Exhaust of Adiabatic Diesel Engines, by M. M. Barley, Jul. 1985, DOE/NASA/50194-43 NASA TM-86953. This study compared the efficiencies of turbocharged, turbocharged-after cooled engines, turbocharged-turbocompounded and turbocharged-turbocompound-after cooled. It also compared the costs and efficiencies for three methods of auxiliary power derived from the waste heat, namely steam Rankine, organic Rankine and Brayton cycles. All of this recent work involves turbine auxiliaries, i.e. in a turbocharged-turbocompounded diesel engine, the diesel exhaust passes first through a hot turbine connected by a shaft to a turbocompressor and then the hot gas passes through another hot turbine connected through a gear train to the crankshaft. Analysis of this report shows that none of these alternatives are presently economical with respect to a standard diesel truck engine with turbocharging and aftercooling. That is to say that the fuel saving cannot pay for the increased capital cost of the compound engine designs. Likewise, older compound diesel engine designs employing compounding by auxiliary pistons have never proven economical. This is in the most part because designs proposed in the past have lost far more power per unit volume of cylinder than they gain in specific fuel consumption or again the fuel saving could never justify the increased capital cost.
Attention is also directed towards A Review of the State of the Art and Projected Technology of Low Heat Rejection Engines, 1987, National Research Council, National Academy Press, Washington, DC. This reference discusses various approaches towards boosting efficiency by achieving lower heat rejection rates.
Since the advent of the internal combustion engine many evolutionary changes in design have taken place to improve its performance and reliability. In the past twenty years the subject of adiabatic engines has received a great deal of attention. Initial prediction of gains to be achieved by adiabatic engines have not been borne out and in 1985 the National Research Council of the United States of America through its Energy Engineering Board started a review of the state of the art of low heat rejection internal combustion engines (An Assessment of the Performance and Requirements for "Adiabatic" Engines, J. Zucchetto, P. Meyers, J. Johnson, D. Miller, Science, Vol. 240, May 27, 1988). The conclusions were that for a cylinder with isothermal walls 4% and 8% improvements in fuel efficiency could be obtained. A good report on the economics of low heat rejection engines and methods of heat recovery from the exhaust gases is given by the DOE/NASA report referred to earlier.